Petaluma Regional Library is at 100 Fairgrounds Drive. Free parking is available at the library or the fairgrounds next door.

To report an otter sighting or to check out the Otter Spotter, click here.

River otters are making a comeback in Sonoma County and across the Bay Area thanks in part to improved water quality and habitat restoration projects, according to ecologists.

In recognition of last week’s World Otter Day, local otter fans are hosting a Saturday lecture and series of kids’ activities at the Petaluma Regional Library from 11 a.m. to 1 p.m.

The message of the lecture will be one of resiliency and recovery, said Megan Isadore, executive director of the River Otter Ecology Project, which is hosting the North Bay event. A second event is also taking place in the South Bay.

Pollution of the San Francisco Bay and surrounding waterways decimated river otters during the middle of the last century, Isadore said, causing them to retreat to less polluted areas.

Sonoma County never completely lost its otter populations but they certainly suffered during that period, she said.

Significant improvement in Bay water quality and surrounding rivers and creeks and a resulting boost in subsequent decades in stocks of fish and other animals otters feed on helped the species repopulate streams throughout the Bay Area, she said.

Otters are cute, but also ferocious predators that can only thrive if their food chain — including fish, crayfish and amphibians — is healthy, as well, she said.

“Their presence tells us that we are doing something right,” she said.

The organization doesn’t have precise population figures, but the Otter Spotter feature on its website suggests they’re doing well here.

“It appears to be a pretty healthy population,” she said.

That’s the impression that Steve Brady, an environmental specialist with the City of Santa Rosa, gets from regular reports of sightings by residents.

“We used to get a couple of reports a year. Now we’re getting them all the time,” Brady said.

Sightings in Santa Rosa, Piner and Brush creeks are common, he said.

Brush and Santa Rosa creeks have benefited from major riparian restoration projects in the 1990s and 2000s, Brady noted. The restoration of Colgan Creek is ongoing.

The city also coordinates the removal of tons of trash, mostly through the work of volunteers, from area creeks annually, he said.

You can reach Staff Writer Kevin McCallum at 707-521-5207 or kevin.mccallum@pressdemocrat.com. On Twitter @srcitybeat.